This project concerns certain aspects of genome organization in Drosophila melanogaster. A group of repeated sequences has been analyzed which are homologous to the ribosomal insertion, i.e., the region of DNA which interrupts about half of all ribosomal genes that are located on the X chromosome of Drosophila. Such seqences have been found earlier and are located primarily in the chromocenter of salivary gland chromosomes. A collection of DNA molecules containing sequences homologous to the insertion has been isolated by molecular cloning. Several of these clones were analyzed. The insertion-homologous sequence is interspersed with other regions called flanking sequences. Five different flanking sequences were studied, showing that they are representatives of moderately repetitive sequences.